N = int(raw_input())
A = map(int, raw_input().split())

total = sum(A)

c = 0
sub_total = 0
mn = total

while sub_total < total:
    diff = 0
    B = [0] * (max(N, c * 2))
    for i, v in enumerate(A):
        B[i] = v

    for i in xrange(c):
        t = B[i] - (i + 1)
        if t > 0:
            diff += t

    for i in xrange(c, c * 2 - 1):
        if i >= len(B):
            print i, len(B), c
        t = B[i] - (2 * c - i - 1)
        if t > 0:
            diff += t

    for i in xrange(c * 2 - 1, N):
        diff += B[i]

    mn = min(diff, mn)

    c += 1
    sub_total += c + (c - 1)

print mn